With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
The performance of gas turbine engines, whether measured in terms of efficiency or specific output, is improved by increasing the turbine gas temperature. It is therefore desirable to operate the turbines at the highest possible temperatures. For any engine cycle compression ratio or bypass ratio, increasing the turbine entry gas temperature produces more specific thrust (e.g. engine thrust per unit of air mass flow). However as turbine entry temperatures increase, the life of an un-cooled turbine falls, necessitating the development of better materials and the introduction of internal air cooling.
In modern engines, the high-pressure turbine gas temperatures are hotter than the melting point of the material of the blades and vanes, necessitating internal air cooling of these airfoil components. During its passage through the engine, the mean temperature of the gas stream decreases as power is extracted. Therefore, the need to cool the static and rotary parts of the engine structure decreases as the gas moves from the high-pressure stage(s), through the intermediate-pressure and low-pressure stages, and towards the exit nozzle.
FIG. 2 shows an isometric view of a typical single stage cooled turbine in which there is a nozzle guide vane in flow series with a turbine rotor. The nozzle guide vane includes an aerofoil 31 which extends radially between inner 32 and outer 33 platforms. The turbine rotor includes a blade mounted to the peripheral edge of a rotating disc. The blade includes an aerofoil 32 which extends radially outwards from an inner platform. The radially outer end of the blade includes a shroud which sits within a seal segment 35. The seal segment is a stator component and attached to the engine casing. The arrows in FIG. 2 indicate cooling flows.
Internal convection and external films are the prime methods of cooling the gas path components—airfoils, platforms, shrouds and shroud segments etc. High-pressure turbine nozzle guide vanes (NGVs) consume the greatest amount of cooling air on high temperature engines. High-pressure blades typically use about half of the NGV flow. The intermediate-pressure and low-pressure stages downstream of the HP turbine use progressively less cooling air. Cooling requirements often result in complex arrangements and geometries which are difficult to cast which require additional design and trial periods.
In addition to being able to withstand the elevated operational temperatures within the gas turbine, the turbine parts are also required to withstand the stress they are subjected to. The levels of stress will clearly vary according to the part it is generally true that increasing the stiffness of a component is conventionally achieved by increasing wall thickness or including strengthening elements such as webs or flanges. However, there is a direct correlation between increasing the thickness of a wall and the weight of a component which provides the component designer with a challenge to maintain the required stiffness in a component whilst reducing the weight as much as possible.
This invention seeks to provide an air cooled component which provides an increased stiffness for a given weight of material.